This invention relates generally to active suspensions of vehicles, particularly land motor vehicles. More particularly, the invention concerns a novel active suspension system of a vehicle which is designed to improve the riding comfort of the vehicle through consideration of the up-and-down (vertical) acceleration of the mass above each suspension spring, the relative vertical displacements and relative displacement velocities of the masses above and below each suspension spring
In an automotive vehicle, a so-called active suspension system installed between the vehicle frame and the wheel assemblies has been known in the prior art (as disclosed, for example, in Japanese Patent Laid-Open Publication No. 213510/1984). In an active suspension system, the damping characteristics, provided by the gasliquid fluid springs of the suspension units in which these springs are used, are variably controlled in accordance with the relative displacement (mutually relative displacement quantity) in the up-and-down (vertical) direction of the wheel axles and the vehicle frame and the rate of variation with time of this relative displacement, that is, relative displacement velocity in the vertical direction.
In this known system, the nature of the road surface, i e., whether it is smooth or whether it is rough, is determined from the vertical-direction relative displacement and the relative displacement velocity of the wheel axles and the vehicle frame, and the damping force of the gas-liquid spring in each suspension unit is controlled to be low if the road surface is smooth and to be high if the road surface is rough. In the case where the road surface is judged to be smooth, the damping characteristic becomes soft, and ordinary riding comfort can be increased, but there arises a problem in that resonance of the mass above the spring cannot be suppressed, and a bouyant or wafting sensation is strongly felt.
Furthermore, when the vehicle is steered into a turn in the case where the road surface has been judged to be smooth, a rolling motion occurs. If, in order to prevent this rolling, a measure such as switching the damping force to a higher value is taken, the suspension at the time of turning will become stiff or hard, whereby the riding comfort will be greatly impaired.
Still another problem encountered in the above described active suspension system is that the required flow rates of the system fluids with respect to high-frequency vibration becomes extremely high and cannot be delivered without a tremendous increase in the capacities of the air compressor, oil pump, and other components, whereby increases in equipment cost and consumed energy cost cannot be avoided